Engineering of the membrane-like tissue structures with ability to remodel and regenerate is currently an unresolved subject in the field of tissue engineering. Several attempts with minimal success have been made to create functional viable membrane tissues such as heart valve leaflet with the ability to grow, repair, and remodel. Shinoka et al. fabricated single leaflet heart valves by sequentially seeding ovine fibroblasts and endothelial cells on a bioabsorbable polymer composed of a polyglactin woven mesh surrounded by two nonwoven polyglycolic acid mesh sheets. (See Shinoka, T., Breuer, C. K., Tanel, R. E., Zund, G., Miura, T., Ma, P. X., Langer, R., Vacanti, J. P., and Mayer J. E. Tissue engineering heart valves: Valve leafet replacement study in a lamb model. Ann Thorac Surg, 60, 13, 1995). Hoerstrup et al. fabricated a trileaflet heart valve using nonwoven polyglycolic acid mesh, a bioabsorbable polymer, sequentially seeded with ovine myofibroblasts and endothelial cells made using a pulse duplicator in vitro system. (See Hoerstrup, S. P., Sodian, R., Daebritz, S., Wang, J., Bacha, E. A., Martin, D. P., Moran, A. M., Guleserian, K. J., Sperling, J. S., Kaushal, S., Vacanti, J. P., Schoen, F. J., and Mayer, J. E. Jr. Functional living trileaflet heart valves grown in vitro. Circulation, 102, 44, 2000). Sodian et al. constructed trileaflet heart valve scaffolds fabricated from seeding ovine arterial vascular cells on a polyhydroxyoctanoate material. (See Sodian, R., Hoerstrup, S. P., Sperling, J. S., Daebritz, S., Martin, D. P., Moran, A. M., Kim, B. S., Schoen, F. J., Vacanti, J. P., and Mayer, J. E. Jr. Early in vivo experience with tissue-engineered trileafet heart valves. Circulation, 102, suppl III, 2000). Sutherland et al. created autologous semilunar heart valves in vitro using mesenchymal stems cells and a biodegradable scaffold made of polyglycolic acid and poly-L-lactic acid. (See Sutherland, F. W., Perry, T. E., Yu, Y., Sherwood, M. C., Rabkin, E., Masuda, Y., Garcia, A., McLellan, D. L., Engelmayr, G. C., Sacks, M. S., Schoen, F. J., and Mayer J. E. Jr. From stem cells to viable autologous semilunar heart valve. Circulation, 111, 2783, 2005). Drawbacks to the approaches described above include structural vulnerability, short term functionality, and limited mechanical properties of the membrane constructs.
Scaffolds are critical components of the engineered tissues that allow them to be formed in vitro and remain secure in vivo when implanted in a host. Several approaches have been taken to develop scaffolds for tissue membranes. The most widely used method involves biodegradable naturally-derived or synthetic polymers, where the polymer eventually degrades by normal metabolic activity, while the biological matrix is formed. To have viable tissue, the rate of scaffold degradation should be proportional to the rate of tissue formation to guarantee mechanical stability over time. The poor control of enzymatic degradation and low mechanical performance are two major limitations of naturally derived polymers. In contrast, synthetic polymers can be prepared precisely with respect to structure and function. However, most of them produce toxic chemicals when they degrade in vivo, and due to lack of receptor-binding ligands, they may not provide a good environment for adhesion and proliferation of cells.
Another option for creating scaffolds is to use decellularized xenogenic tissues, which has some advantages over polymeric materials. Decellularized tissues provide a unique scaffold, which is essentially composed of extracellular matrix (ECM) proteins that serve as an intrinsic template for cells. However, the process of decellularization cannot completely remove the trace of cells and their debris. These remnants not only increase the potential of an immunogenic reaction, but also result in increased tissue susceptibility to calcification.
Another, albeit less developed, strategy involves creating a scaffold with completely biological matrix components. This approach has advantages over using polymeric materials or decellularized xenogenic tissues. For example, large amounts can be produced from xenogenic sources, which can readily accommodate cellular ingrowth without cytotoxic degradation products. However, this strategy is restricted due to mechanical fragility of the scaffold and the low potentials for creating complex tissue structures.
Thus, a continuing need exists for a tissue construct that is strong enough to resist forces that exist inside a body, while possessing biocompatible surfaces.